The Mahavagga (part of the Vinaya collection) includes accounts of Gautama Buddha’s and the ten principal disciples’ awakenings, as well as rules for ordination, rules for reciting the Patimokkha during uposatha days, and various monastic procedures....

Mahavagga, Khandaka 2, Chapter 10

At that time in a certain district on the day of Uposatha the young Bhikkhus, who had assembled first, thought: 'The Theras do not come yet,' and went away. The Uposatha service was held after the right time.

They told this thing to the Blessed One.

“I prescribe, O Bhikkhus, that on the Uposatha day the Theras ought to assemble first.”